The simplest way to do something like this would be to use the index function. The index function counts the position the current line is depending on the partition. I used the following calculation below to get the desired result (sample workbook attached).

Formula:

CASE [!Range Selector]

WHEN "Low" then

INDEX() <= 15

When "Med" then

INDEX() >= 16 and INDEX() <= 30

When "High" then

INDEX() >= 31

END

Please keep in mind that it is important to know how the table calculations are being computed.